The present invention generally relates to a laser welded joint and method and, more particularly, to a laser welded joint and method especially for fusion welding overlapping zinc coated metal plates.
Galvanized steel is used extensively in the automotive industry because of its low cost, availability and corrosion resistive properties. It is the preferred material for auto-body manufacturing which is generally joined by fusion welding. However, fusion welding of coated zinc steel presents a challenge for manufacturing engineers because of the low melting point of zinc (907xc2x0 C.) and the high melting point of steel (1530xc2x0 C.). In lap welding of galvanized steel, the zinc layer evaporates violently at the weld interface resulting in undesirable high porosity in the welded joints.
One prior art solution uses a constant gap between overlapping steel plates. The gap is designed to allow the explosive zinc vapors to vent through the gaps resulting in acceptable weld joints. However, in commercial manufacturing environments, maintaining such a constant gap is difficult. Thus, the use of constant gap for fusion welded galvanized steel assemblies has not been found to be commercially feasible. Another solution has been to use an elongated laser beam to keep the melt pool heated for a longer period so that zinc vapors get enough time to escape from the weld zone. However, this method involves additional hardware and puts a constraint on the cost.
Thus, there is a need for a laser welded joint and method that joins overlapping layers of galvanized steel which limits the explosive behavior of zinc vapors, reduces porosity in the weld joint and produces a strong, sound and repeatable weld joint.
The present invention seeks to overcome the foregoing drawback by providing, in one aspect, a method for laser welding a pair of overlapping galvanized metal plates together. At least one of the pair of overlapping metal plates has a protective layer of metal. The method includes sandwiching a foil member and/or coating of appropriate material which will alloy with zinc to avoid violent evaporation between the pair of overlapping metal plates; welding the pair of overlapping metal plates; and melting the layer of metal and the foil member to form an alloy that is disposed between the overlapping metal plates.
In accordance with another aspect of the invention, a welded joint is formed by a laser beam welder. The joint includes a pair of overlapping metal plates. At least one of the pair of overlapping metal plates has a protective layer of zinc. Additionally, a metal filler member is disposed between the pair of overlapping metal plates. The metal filler member, the protective layer of zinc and the pair of overlapping metal plates are heated by the laser beam welder. The laser beam welder vaporizes the protective layer of zinc, melts the metal filler member and welds the pair of overlapping metal plates together. The vaporized layer of zinc is substantially trapped and absorbed in the molten metal filler member to form an alloy. The alloy is disposed between the pair of overlapping metal plates.
It is an object of the present invention to provide a weld joint and method that welds a pair of overlapping galvanized metal plates together and provides a metal filler member that sets a gap between the pair of galvanized metal plates and which when heated melts to trap and absorb vaporized zinc to form an alloy disposed between the pair of galvanized metal plates. The metal filler can be applied as foil, paint, slurry or wire to name a few.
It is another object of the present invention to provide a weld joint and method that lap welds a pair of galvanized metal plates together and provides a metal filler that melts when heated by the laser welder to trap and absorb vaporized zinc to reduce porosity in the welded joint.
These and other objects will become more apparent from reading the specification, drawings, and claims.